1. Field of the Invention
The present invention relates to a power steering system of a vehicle and, in particular, to a steering force control apparatus for a power steering system in which the steering torque of an input shaft which performs steering is calculated by detecting a physical quantity factor which varies depending on a vehicle travelling condition and a steering condition. The steering torque is made variable in accordance with a change in the physical quantity factor.
2. Description of the Prior Art
Prior art steering force control apparatus for a power steering system of this type are disclosed, for example, in Japanese Patent Publication No. 53-8086, Japanese Patent Laid-Open Publication No. 50-59930 and U.S. Pat. No. 3,877,540.
In these steering force control apparatus, plungers are provided as a steering reaction force generating mechanism in which the plungers abut a pair of projections formed symmetrically on an input shaft respectively in both sides of a direction of rotation of the input shaft to thereby generate a force couple in a forward direction of rotation and a reverse direction of rotation of the input shaft. Thus, during travelling of the vehicle at high speeds, a steering reaction force is applied to an input and an appropriate steering force is obtained in response to a travelling condition of the vehicle.
Furthermore, a control apparatus which is applicable to the steering force control apparatus of a power steering system is disclosed, for example, in U.S. Pat. No. 4,730,687 (Japanese Patent Laid-Open Publication No. 61-155058) which was proposed previously by the Applicant's company.
In this prior art apparatus, a variable orifice is placed in a supply passage which supplies a predetermined flow rate of working fluid supplied from a hydraulic pump. The position of the variable orifice is controlled by a control signal from a controller which sets a steering force based on a detection signal of a detecting means for detecting a physical quantity factor which is varied depending on travelling and steering conditions of a vehicle. Thus, the supply passage is split into two passages by the variable orifice in accordance with the control signal. The working fluid of one of the two passages is supplied through a passage change-over valve to a steering assisting force generating mechanism for generating a steering assisting force. The working fluid of the other passage is supplied through the passage changeover valve to a steering reaction force generating mechanism for generating a steering reaction force. As a result, a steering force control apparatus of a power steering system is provided in which a large change in an input/output characteristic is obtained, the sensitivity for steering feeling is sharp, and the consumption of energy is relatively low.
However, in the prior art steering force control apparatus of a power steering system, in the steering reaction force generating mechanism, accommodating holes for slidably accommodating the plungers which generate a force couple in a reverse direction with respect to a steering direction of the input shaft are formed as bottom-provided holes and, in particular, it is difficult to grind a deep endmost portion in which the plungers slide. As a result, a problem is involved in that smooth operation of the plungers is not insured.
To solve this problem, it is considered to make the plunger accommodating holes in the form of through bores, and to provide flanges serving as the stoppers at an end of the plungers. However, in this case, another problem is caused in that since the flanges are formed on an outer peripheral surface of the plungers, the outer peripheral surfaces of the flanges cannot be ground accurately.
Furthermore, in the prior art control apparatus applicable to the steering force control apparatus, since the variable orifice is structured in that a spool for controlling the flow rate of the split passages is operated by a proportional, solenoid operated valve in proportion to a control signal from a controller and is directly controlled with respect to its sliding position, it is impossible to increase a driving force for the sliding position control of the spool. As a result, there is a problem in that an accurate position control is difficult when the sliding resistance of the spool is increased due to adherence of dust or the like to the sliding surface of the spool.